


Biological Thesis

by Askellie (NadaNine)



Category: Undertale (Video Game)
Genre: Daddy Kink, Exotic Cocks, M/M, Mating Cycles/In Heat, Multi, Sibling Incest, Tentacle Dick, Threesome - M/M/M
Language: English
Status: Completed
Published: 2017-04-03
Updated: 2017-04-03
Packaged: 2018-10-14 08:43:05
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,171
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/10532913
Author URL: https://archiveofourown.org/users/NadaNine/pseuds/Askellie
Summary: “You can do a proper biological thesis on it later,” G remarked dryly, giving his brother a pointed stare when it looked like he might want to pull out his research notes. “Right now, I think Sansy needs your attention.”G!Papyrus and G!Sans are almost like father figures, or at least older siblings. Surely that means they'll help Sans out in his time of need.





	

_You can do this. He’ll understand. Even if he’s not your brother, he’s still Papyrus. He’ll want to help you._

Sans tried to draw in a steadying breath and only ended up wheezing unevenly, watching his hand tremble above the door release button before he forced himself to press it. The hydraulics hissed as the entry to the lab slid open, and he crossed the threshold with short, faltering steps.

The lab was brightly lit even though it was well after midnight, and as expected there was a lone figure bending thoughtfully over one of the tables, examining a set of blueprints. At the sound of the door, the figure straightened, turning to face Sans with a befuddled blink that quickly brightened with recognition.

“Sans! You’re up late…or is it early? Hmm…” Papyrus squinted at the nearest computer screen, absently scratching at his chin in a gesture that wasn’t at all like the Papyrus Sans knew, but was so damn familiar. It was so jarring, watching those elegant fingers tracing the deep crack that ran from the sharp angle of his jaw to the bottom of his eye-socket the way Gaster used to do when faced with a particularly perplexing problem.

He hadn’t yet worked up the courage to ask how it happened – why this version of himself and his brother had somehow merged with the old man. Both of them bore his distinctive marks, the scars, the holes in his hands, and seemed to have inherited a portion of his personality. This Papyrus was much more reserved; thoughtful, scholarly, but with reassuring flashes of Sans’s own brother’s confidence and enthusiasm. Stranger still was the way they’d all but adopted Sans, assuring him that they would take care of him until they could find a way to return him to his own universe.

For the first time in a long time, Sans felt…safe. Cared for. Protected. The last time he’d truly felt that way was with Gaster himself when the old man had summarily adopted Sans and his brother, taking them from their life on the streets, giving them a roof over their heads, food, education…Sans had always thought the old man was a kook. He was a relic of the old days, before monsters had been trapped in the underground, before the world had turned to kill or be killed. Sans had never understood where that selfless generosity had come from, and hadn’t seen it again after Gaster had died. Not until now, at least.

“Was there something you needed?” Papyrus asked, skill tilted quizzically in Sans’s direction. Even without receiving a response he was already stripping off his lab coat, willing to step away from his work to give Sans his full attention. Even if that was exactly what Sans had hoped for, he felt ashamed and unworthy of it. Papyrus was working to fix the machine to get him home. Sans shouldn’t be bothering him, especially not with this.

“U-um…” Every shred of confidence he’d managed to gather fled as he stared up – and up, god, he was so tall – into the warmly concerned face of this universe’s Papyrus. He felt small and stupid and downright childish, suddenly regretting leaving his bed at all. He could just make some lame excuse and leave, but his feel felt like they were rooted to the floor and there was an awful, conflicted coiling happening in his groin.

It only got worse as Papyrus approached, gently taking Sans’s face between his hands and scrutinising his expression. “Did you have another nightmare? Would you like for me to come out and talk with you a while? I can heat some milk for you.”

This Papyrus was so touchy. Gaster had been that way too – always spawning additional hands to map out new discoveries by touch. Sans’s instincts wanted him to flinch away from the potential threat, but an even more compelling impulse wanted him to relax, to close his eyes and turn his face into those punctured palms and run his tongue around the edges of the holes-

“Please,” Sans whispered and immediately hated the weak, reedy whine of need in his voice. God, he was pathetic. He was doing this all wrong. He should be dragging Papyrus down to his level and pinning him to the floor and ravishing him with a show of strength. That was how it was done in his universe. He grabbed Papyrus’s wrists, willing his hands to stop trembling. “I…I need to…“

“He’s in heat, bro,” a lazy drawl piped up from behind Sans, startling them both violently. The door hadn’t opened to announce their new visitor.

“Brother! What have I told you about teleporting into the Lab,” Papyrus scolded, his attention turning away from Sans, leaving him feeling momentarily bereft. Sans couldn’t help directing a scowl of his own at the interloper; his own cross-universe counterpart, who was leaning nonchalantly against the door frame, a cigarette balanced on the corner of his smirk. It was even more jarring seeing a warped reflection of his own features bearing those twin cracks, though unlike himself, this Sans had flat teeth in a perfect, even smile – no gold replacement necessary. He’d told Sans to call him G after Sans kept tripping on his own name. Papyrus had given his brother a weird look at that, but hadn’t argued.

“I was just keeping an eye on our little brother here,” G said with a playful grin, making Sans’s glower deepen. His counterpart had been lucky enough to inherit a portion of Gaster’s height along with his markings, standing head and shoulders about Sans. The ‘little’ brother jokes had been persistent and annoying. “And I wanted to be nearby in case you needed any help taking care of his little problem.”

“Prob-OH!” The first comment G had made must have come back to Papyrus belatedly, and he looked at Sans with comically widened sockets. “Oh goodness, I didn’t realise…I mean, I apologise, Sans, I should have been more attentive. It seems quite obvious in retrospect, especially since you are…dripping…”

Sans flushed a brilliant shade of crimson, and G laughed uproariously. It wasn’t like Sans could help it. His heat must have started some time in his sleep because he’d woken up with his pussy already slick and throbbing and desperate. The crotch of his shorts were already absurdly wet, and he could feel something trickling down the inside of his femur. He didn’t dare look at the floor, too mortified to see what kind of puddle he might have formed.

Sans covered his face with his hands, making a muffled groan of humiliation only for Papyrus to gently pry them away, petting Sans’s skull reassuringly. “Now Sans, there’s no need for embarrassment. This is a perfectly natural biological response, and I know you couldn’t have predicted such a thing given how uncommon heats are in your world…goodness, that’s really quite flattering! You must feel very safe here for it to have triggered at such a time-”

“You can do a proper biological thesis on it later,” G remarked dryly, giving his brother a pointed stare when it looked like he might want to pull out his research notes. “Right now, I think Sansy needs your attention.”

“Of course!” Papyrus agreed, somewhat sheepishly. He knelt down to Sans’s height, hoping to put the smaller skeleton at ease. “Tell me what you need, Sans. I believe I may have some hormone suppressants which should help relieve your symptoms, or if you like there are several excellent professionals in town we can contact to-”

Reaching the absolute limit of his patience, Sans threw his arms around Papyrus’s neck in what was practically a stranglehold and smashed their mouths together with a painful clack of teeth. Papyrus’s mouth had already been open mid-word, and Sans quickly shoved his tongue inside the taller’s mouth and began a furious exploration.

G snorted, greatly amused. “You’re pretty dense, bro. He wants you to help.”

It took a long minute for Papyrus to recover his wits, making Sans fear for a moment he’d fucked up horribly, but soon Papyrus’s tongue curled tentatively around his own and then responded with matching enthusiasm. Sans groaned with relief as he was abruptly crushed to papyrus’s chest, his mouth vigorously claimed before Papyrus pulled back enough to give them both space to breathe.

“Me?” Papyrus echoed, sounding oddly surprised.

“My Papyrus helps me with my heat,” Sans murmured, not quite able to meet Papyrus’s gaze, eyelights downturned to the floor as his face burned with heat. “And before him…Gaster…”

It was almost a pity he didn’t look up to see the curious transformation that took place on Papyrus’s face. His eye sockets narrowed, and the softness of his expression turned sharper, more calculating as he eyed the small skeleton pressed against him. He gave his brother a meaningful look that G returned with a crooked smirk.

“I see,” Papyrus said, scooping Sans up in his arms and lifting him easily. Sans felt positively tiny in his grasp, overpowered and overwhelmed even as Papyrus settled in the nearest chair, shifting Sans to straddle his legs, facing outwards. His spine was pressed to Papyrus’s chest, and the wetness of his shorts immediately began to seep into Papyrus’s trousers. The drenched fabric felt rough against his summoned pussy. “Then as your current guardian, it is my responsibility to take care of you…Will you be doing more than watching, brother?”

“If he wants,” G replied so mildly it might be easy to assume he really didn’t care one way or the other. The predatory glint in his eyes spoke otherwise, though, as he approached. At this height, Sans’s face was level with G’s crotch, and there was a telling shift of movement through the taller skeleton’s tight, dark jeans. Sans’s eyelights were already blown out and hazy, and deliriously he pressed his face forward, nuzzling against the promising swell of his counterparts magic. G chucked. “Guess that’s a yes. We gonna take care of him, brother?”

“Indeed we shall, brother.”

Sans mewled, his body shuddering with memories as both their words were overlaid with the sibilant consonants of Gaster’s peculiar speech. They did that sometimes, their words becoming oddly blurred and syncing as if they were one person speaking with two different bodies. It was eerie and yet compelling, and with his mind clouded with heat, Sans only felt more aroused. He tried to paw desperately at G’s belts, utterly confounded by the complicated loops and buckles that seemed completely unrelated to actually holding his pants in place. Thwarted, Sans let out a frustrated whine. Papyrus laughed against his skull, deep and resonant, and reached over Sans’s shoulders to accomplish the task for him.

“I agree, Sans. You fashion choices are absurd, brother,” Papyrus tutted, deftly unhooking the various clasps and finally gaining access to the button of G’s fly. He reached in, long, pale fingers gently pulling free a long, writhing appendage that definitely wasn’t the traditional cock Sans had been expecting. G’s magic was tinted yellow, and Papyrus’s was a deep, steady green, but this was something else entirely. It was a long, slippery tentacle, dark and oily, already seeping some sort of viscous fluid. G groaned, and as Sans watched its writhing, flexible length dove eagerly into the hole in Papyrus’s palm. It pumped in and out excitedly, making G hiss and Papyrus loose an uncharacteristic growl of desire before he pulled his hand free. The tentacle tried to follow, and received a light smack for its efforts.

“Enough of that. We have a more important matter to attend to.” His arms wrapped carefully around Sans, holding him steady as an undulating motion beneath him threatened to unseat him. He could feel something wriggling beneath him, groping sinuously at his tailbone, and had no doubt that if he looked down he would see a matching appendage bursting free of Papyrus’s trousers. His mouth watered, and he ground down on the slippery thing enticingly. God, it felt so huge. He wanted that in him right now.

“Looks like he wants it bad,” G noted, his hands holding either side of Sans’s head so that his (cock? tentacle?) could lick a slippery path against Sans’s cheekbone, teasingly circling his eye socket and the edges of his mouth. Sans let his own tongue slip out, and it met the appendage in a filthy, wet entwinement that was almost like a kiss. He tasted like salt, and a little of the acrid sweetness of his cigarettes along with a thick, sexual musk. 

Papyrus’s hold on him tightened, making Sans squeak appreciatively as his ribs creaked. “Indeed. We will try to be gentle with you, Sans, but I hope you understand. My brother and I can get very…caught up in new discoveries.”

Sans just nodded frantically as his shorts were torn off him, hitting the ground with a wet slap as Papyrus positioned him at a better angle.


End file.
